1. Technical Field
This application relates to wireless communication techniques in general, and to a system and method of transmit power control for a mobile wireless device with multiple operating modes of the antenna in particular.
2. Description of the Related Art
An antenna is used in a mobile communication device for picking up received signals and for radiating transmitted signals. For transmitting purposes, the radiated radio frequency field includes “Far Field” and “Near Field” components. The far field is the radiated field that is useful for wireless communication. The transition from the near field to the far field is gradual but a practical definition is that the near field is dominant at distances less than twice an antenna's largest dimension squared divided by the wavelength from the antenna. Conversely, the far field refers to the field generated by the antenna at distances beyond twice an antenna's largest dimension squared divided by the wavelength.
The near field is close to the user's body, and also close to the circuitry within the wireless device, both of which may result in various side effects. One of these side effects is radiation absorption to the human user, measured by the Specific Absorption Rate or SAR. SAR is the measurement of the amount of radiation absorption by the human body. SAR is usually calculated in watts per kilogram or milli-watts (mW) per gram.
Different countries have different regulatory requirements for SAR. For example, in North America, the SAR of a handheld wireless communication device might be regulated to not exceed the 1.6 mW/g limit while the device is held at a human head. It is a challenge to design an antenna and its surrounding structure of a wireless communication device to generate a strong far field, while also minimizing SAR, as these may be conflicting requirements.
Certain designs may utilize multiple operating antenna positions to satisfy the conflicting requirements. For example, a whip antenna can have two operating positions—an extended position and retracted position; a flip mobile phone can have open and close positions, and the antenna can further be extended and retracted in combination with the open and close positions. Each of the positions may have different effects to the SAR of the user and to the antenna gain in the far field. This may not be an acceptable solution, as improving the situation in one field may be done at the expense of the situation at the other field.
The SAR is proportional to the transmitted power generated by a device. The transmitted power is typically regulated by a transmit power control unit in the device. A maximum allowed transmit power is usually set to a given value in a transmit power control unit so that the SAR cannot exceed the regulatory limit.